Methods and compositions for increasing solubility of azole drug compounds that are poorly soluble in water

ABSTRACT

The combination of any two of a polyol, a polyol ether, and a low carbon organic alcohol provides a synergistic effect on the solubility of azole compounds, such as metronidazole, in aqueous fluid.

FIELD OF THE INVENTION

The present invention pertains to the field of increasing solubility of chemical compounds in aqueous fluids.

BACKGROUND OF THE INVENTION

Many chemical compounds, particular chemical compounds that are useful in pharmacologic applications, are poorly soluble in water. Such drugs may be classified according to USP-NF as being sparingly soluble, slightly soluble, very slightly soluble, or insoluble in water. Many of these compounds are also poorly soluble in oils. An example of such a chemical compound includes the azole family of drugs, which family includes metronidazole, fluconazole, ketoconazole, itraconazole, miconazole, dimetridazole, secnidazole, ornidazole, tinidazole, carnidazole, and panidazole.

In many situations, such drugs may be formulated as a suspension, in which case the lack of solubility of the drug in water and in pharmaceutical oils does not typically have a negative impact on bioavailability. However, in situations where a solution of the drug is desired, or where an emulsion containing the drug in solution either in the hydrophilic or lipophilic phase of the emulsion is desired, the lack of solubility of the drug in water and oils provides a significant obstacle to obtaining desired concentrations of the drug.

The need to increase the aqueous solubility of poorly water soluble drugs that are also poorly soluble in oils is especially critical when formulating an emulsion, such as a topical cream, lotion, or ointment, containing such a drug. In this case, in order to obtain a sufficiently high concentration of the drug in the emulsion formulation as a whole, the concentration of the drug in the aqueous phase must be sufficiently elevated to overcome the lack of partitioning of the drug into the oil phase.

Metronidazole is an antimicrobial drug that is administered systemically for treatment of infections with anaerobic bacteria and protozoans, such as Trichomonas, Entamoeba, and Giardia. It is also used topically to treat bacterial vaginosis and various forms of acne, including acne rosacea. The aqueous solubility of metronidazole in water at room temperature is only about 0.87% w/w. Additionally, physically stable solutions of metronidazole in water that can withstand exposure to cold temperatures encountered during shipping are limited to about 0.7% w/w metronidazole. However, for many topical applications, a concentration of 1.0% or higher is desired.

The lack of solubility of such drugs, and the inability to obtain sufficiently high concentrations of drugs in solution in pharmaceutically acceptable carriers, is a serious problem in the formulation of topical therapeutic products for the treatment of medical conditions affecting the skin or mucosa. Such lack of solubility is additionally a concern in the formulation of medications for parenteral administration and for oral liquids that are often used for children and geriatric patients. Accordingly, a significant need exists for a method to increase the solubility of drugs that are poorly soluble in water, and especially those, like azole drugs such as metronidazole, that are also poorly soluble in pharmaceutical oils.

DESCRIPTION OF THE INVENTION

It has been unexpectedly discovered that the combination of any two of a polyol, a polyol ether, and a low carbon organic alcohol provides a synergistic effect that produces an increase in the aqueous solubility of poorly water soluble compounds such as drugs, including poorly water soluble compounds, such as azole drugs.

As used herein, the term “poorly soluble” when referring to a chemical compound in relation to its solubility in water or an oil means a chemical compound that is sparingly soluble, slightly soluble, very slightly soluble, or insoluble in water or an oil, as defined in U.S. Pharmacopeia and National Formulary (USP-NF). According to this definition, solubility is stated in terms of the parts of the solvent needed to dissolve one part of the solute. A compound that is sparingly soluble in a particular solvent, such as water, requires 30-100 parts of the solvent to dissolve one part of the compound. A compound that is slightly solvent requires 100-1000 parts of the solvent. A compound that is very slightly soluble requires 1000-10,000 parts of the solvent. A compound that is insoluble requires more than 10,000 parts of the solvent to dissolve one part of the solute.

As used herein, the term “polyol” is synonymous with “polyhydric alcohol” and refers to an alcohol that contains more than one hydroxyl group. Examples of polyols include polyether glycols, propylene glycol, and sugar alcohols.

As used herein, the term “polyol ether” refers to an alcohol that contains more than one hydroxyl group and an ether group. Examples of polyol ethers include diethylene glycol monoethyl ether (ethoxydiglycol) (Transcutol®, Gattefosse Corporation, Paramus, N.J.), ethers of pentaerythritol, ethers of alkylene glycol, ethers of a fatty alcohol, and ethers of a sugar.

As used herein, the term “low carbon organic alcohol” refers to an alcohol having the formula RCH₂OH, wherein R is either H or is a straight or branched alkyl chain of 1 to 7 carbons, or having a ring structure directly connected to a hydroxyl group or connected to a hydroxyl group by a carbon. Examples of low carbon organic alcohols include alkyl and aryl alcohols such as ethyl alcohol, propyl alcohol, isopropyl alcohol, phenol, and benzyl alcohol.

An example of such a poorly water soluble compound that is poorly soluble in pharmaceutical oils is the drug metronidazole, a member of the azole family of medications. This drug is utilized as an illustration of the invention. However, it will be understood that the description of the invention herein pertains not just to metronidazole but to all members of the azole family of medications, for example, fluconazole, ketoconazole, itraconazole, miconazole, dimetridazole, secnidazole, ornidazole, tinidazole, carnidazole, and panidazole.

In accordance with the invention, a polyol and a polyol ether, a polyol and a low carbon organic alcohol, a polyol ether and a low carbon organic alcohol, or a polyol, a polyol ether, and a low carbon organic alcohol are combined in an aqueous fluid with a poorly water soluble chemical azole compound, such as metronidazole to form an aqueous solution. The total concentration of the polyol and the polyol ether, the polyol and the low carbon organic alcohol, the polyol ether and the low carbon organic alcohol, or the polyol, the polyol ether, and the low carbon organic alcohol that is combined in the aqueous fluid is that which is sufficient to provide a synergistic increase in the solubility of the azole chemical compound in the aqueous fluid in the absence of any two or more of a polyol, a polyol ether, and a low carbon organic alcohol.

The absolute and relative concentrations of the polyol, polyol ether, and low carbon organic alcohol in the aqueous fluid may be varied, if desired, in order to obtain a particular increase in solubility of the azole compound in the aqueous fluid. It is expected, however, that any amount and any ratio of two or more of a polyol, polyol ether, and low carbon organic alcohol will result in a synergistic increase in solubility of azole compounds in the mixed solvent system compared to the solubility of the azole compounds in an aqueous fluid lacking two or more of a polyol, a polyol ether, and a low carbon organic alcohol.

The aqueous fluid may contain, in addition to the above components, additional components such as, but not limited to, additional solubility enhancing agents such as one or more of a cyclodextrin, niacin, and niacinamide, gelling agents such as a carbomer or a cellulosic polymer, preservatives, chelating agents, pH adjusting agents, and buffers.

The aqueous solution of the invention may constitute the internal aqueous phase of a water-in-oil emulsion or the external aqueous phase of an oil-in-water emulsion. Methods of making such emulsions are well known in the art.

The solutions of the invention, including gels, may be used for the topical treatment of dermatologic or mucosal disorders that are responsive to therapy with azole compounds such as metronidazole. In accordance with the method of treatment of the invention, a stable aqueous solution as described herein containing an azole compound and two or more of a polyol, a polyol ether, and a low carbon organic alcohol is topically applied to skin or mucosal surfaces in need of such therapy. Examples of disorders that are suitably treated in accordance with the invention include inflammatory lesions on skin or mucosa, such as oral or vaginal mucosa, diabetic foot ulcers, and certain infectious diseases that are responsive to topical therapy. A particular disorder that may be treated with the method of the invention is rosacea, also known as acne rosacea.

Preferably, the dissolved concentration of the azole compound in the pharmaceutical formulation of the invention is sufficient so that application once daily is effective to ameliorate the disorder. For example, with metronidazole, concentrations of about 1% or higher provide effective treatment when applied only once daily. At concentrations below 1.0%, it is recommended to apply a metronidazole formulation at least twice daily. The solution containing metronidazole or other azole compound is applied on a daily basis, one or more times per day, for a time sufficient to produce an amelioration or a cure of the disorder. In certain chronic disorders, the solution may be applied one or more times daily for a prolonged period to prevent worsening of the disorder.

The invention is further illustrated in the following non-limiting examples. In the examples, propylene glycol is utilized as a representative polyol, Transcutol® is utilized as a representative polyol ether, benzyl alcohol is utilized as a representative low carbon organic alcohol, and metronidazole is utilized as a representative azole chemical compound. It is understood, however, that other polyols, polyol ethers, low carbon organic alcohols, and azole compounds may be substituted in place of the exemplified propylene glycol, Transcutol®, benzyl alcohol, and metronidazole.

Example 1 Procedure for Determining Solubility of Metronidazole

In the Examples that follow, solubility of metronidazole was determined as follows.

An appropriate amount of each vehicle component was weighted into a 20 ml scintillation vial and the components were shaken until a clear solution was obtained. Metronidazole was then added to the solution and the vials were shaken overnight at room temperature to obtain a saturated solution of metronidazole. The solution was filtered to remove any undissolved metronidazole and the concentration of metronidazole dissolved in the solution was determined by HPLC. The solutions were physically stable at room temperature for at least two weeks, with no precipitation formation.

Example 2 Solubility of Metronidazole in Single Vehicles

The procedure of Example 1 was performed to determine the solubility of metronidazole in a single vehicle solvent selected from water, ethoxydiglycol, benzyl alcohol, and propylene glycol. The results are shown below in Table 1.

TABLE 1 Solvent Metronidazole Solubility % w/w Water 0.87 Ethoxydiglycol 2.47 Benzyl Alcohol 6.23 Propylene Glycol 1.88

Example 3 Solubility of Metronidazole in Multiple Vehicle System Containing Water, a Polyol, a Polyol Ether, and a Low Carbon Organic Alcohol

The procedure of Example 1 was performed to determine the solubility of metronidazole in a multiple vehicle solvent system containing water, ethoxydiglycol, benzyl alcohol, and propylene glycol. The composition of the solvent system is shown below in Table 2.

TABLE 2 Components Amount % w/w Ethoxydiglycol 29.8 Benzyl Alcohol 5.0 Propylene Glycol 10.0 Water 55.2

Based on the solubility of metronidazole in each component of the multi-component vehicle system, the anticipated solubility of metronidazole in this vehicle was calculated to be 1.716% w/w. The calculations are shown in Table 3, where the solubility of each component in isolation times the concentration in the vehicle blend provides the anticipated (additive or calculated) solubility.

TABLE 3 Metronidazole solubility in Fractional individual Anticipated Component Amount component % w/w solubility, % w/w Ethoxydiglycol 0.298 2.47 0.736 Benzyl Alcohol 0.050 6.23 0.312 Propylene Glycol 0.100 1.88 0.188 Water 0.552 0.87 0.480 Total 1.000 1.716

Although, as shown in Table 3, the anticipated solubility of metronidazole in the multiple vehicle system was calculated to be 1.716% w/w, the actual solubility of metronidazole in this multi-component vehicle system was experimentally determined, by HPLC, to be 2.54% w/w, see Table 6, Blend #1, in Example 5 below. The solubility of metronidazole in the multi-component vehicle system was 48.5% higher than expected if the contribution to solubility by each of the components was additive. Thus, the results of this study establish that the combination of water, polyol, polyol ether, and low carbon organic alcohol provides a synergistic effect on the solubility of metronidazole in an aqueous vehicle.

Example 4 Solubility of Metronidazole in Multi-Component Vehicle System Containing Water, a Polyol, a Polyol Ether, and a Low Carbon Organic Alcohol

The procedure of Example 3 was repeated except that the concentration of benzyl alcohol in the aqueous fluid was reduced by 50% and the concentration of water in the fluid was increased accordingly. The composition of the solvent system is shown in Table 4.

TABLE 4 Components Amount % w/w Ethoxydiglycol 29.9 Benzyl Alcohol 2.5 Propylene Glycol 10.1 Water 57.5

Based on the solubility of metronidazole in each component of the multiple vehicle system, the anticipated solubility of metronidazole in this system was calculated to be 1.582% w/w. The calculations are shown in Table 5.

TABLE 5 Metronidazole solubility in Fractional individual Anticipated Component Amount component % w/w solubility % w/w Ethoxydiglycol 0.299 2.47 0.739 Benzyl Alcohol 0.025 6.23 0.156 Propylene Glycol 0.101 1.88 0.190 Water 0.575 0.87 0.500 Total 1.000 1.584

In the solution containing the components shown in Table 4, the solubility of metronidazole in the multi-component vehicle system was experimentally determined, by HPLC, to be 2.07% w/w, see Table 6, Blend #2, in Example 5 below. The solubility of metronidazole in this multi-component vehicle system was 31.0% higher than expected if the contribution to solubility by each of the components was additive.

Example 5 Solubility of Metronidazole in a Variety of Concentrations of the Multi-Component Solvent System

The procedure of Example 1 was utilized to create a variety of multi-component solvent systems containing water, a polyol, a polyol ether, and a low carbon organic alcohol. The anticipated calculated solubility of metronidazole in each system was calculated and the actual solubility of metronidazole in each system was determined by HPLC as described in Example 1. Blends #1 and 2 are the solvent systems of Examples 3 and 4, respectively. The results are shown in Table 6.

TABLE 6 Theoretical Actual Solubility Benzyl Propylene Solubility Solubility Increase Blend # Water Transcutol ® alcohol glycol (% w/w)¹ (% w/w)² %³ 1 55.2 29.8 5.0 10.0 1.71 2.54 48.5* 2 57.5 29.9 2.5 10.1 1.58 2.07 31.0* 3 60.0 30.0 0.0 10.0 1.45 1.66 14.5* 4 65.1 29.9 0.0 5.0 1.4 1.48 5.7 5 65.0 30.0 5.0 0.0 1.62 2.27 40.1* 6 71.0 29.0 0.0 0.0 1.33 1.35 1.5 7 73.1 16.9 0.0 10.0 1.24 1.22 (1.6) 8 74.1 16.0 5.0 5.0 1.44 1.96 36.1* 9 74.5 12.9 2.5 10.1 1.31 1.54 17.6* 10 74.9 23.0 2.1 0.0 1.35 1.66 23.0* 11 81.4 11.1 2.5 5.0 1.23 1.43 16.3* 12 83.9 1.1 5.1 10.0 1.26 1.72 36.5* 13 84.9 0.0 5.1 10.0 1.24 1.66 33.9* 14 89.0 11.0 0.0 0.0 1.04 1.01 (2.9) 15 89.9 0.0 0.0 10.1 0.97 0.93 (4.1) 16 91.5 3.4 5.1 0.0 1.2 1.56 30.0* 17 92.5 0.0 2.5 5.1 1.05 1.20 14.3* 18 95.0 0.0 0.0 5.0 0.92 0.91 (1.1) 19 95.0 0.5 4.5 0.0 1.12 1.44 28.6* 20 97.6 0.0 2.4 0.0 1.00 1.09 9.0 ¹Theoretical (anticipated) solubility is based on the assumption that each solvent's contribution to overall solubility is additive and is calculated from the determined solubility of metronidazole due to each component as shown in Table 1 in Example 2 ²Actual solubility of metronidazole was obtained by HPLC analysis ³Actual solubility minus theoretical solubility divided by theoretical solubility and multiplied by 100. *Solubility increase or (decrease) greater than +/−10% is considered to be significant

The data in Table 6 establishes the synergism obtained by combination of any two of a polyol, a polyol ether, and a low carbon organic alcohol. The highest percentage increases in solubility were obtained in solvent systems that included a low carbon organic alcohol. No increase in solubility above the calculated solubility was obtained when utilizing a solvent system containing water and only one other solvent.

Example 6 Metronidazole 1.5% Gel

An exemplary gel pharmaceutical composition containing metronidazole at a concentration of 1.5% w/w was produced by combining the following components, as shown below in Table 7.

TABLE 7 Components % w/w Metronidazole 1.50 Propylene glycol 5.00 Niacinamide 3.00 Beta-Cyclodextrin 1.00 Methylparaben 0.15 Propylparaben 0.05 EDTA disodium 0.05 Transcutol ® P 15.00 Benzyl alcohol 1.00 Carbopol ® Ultrez 10 0.50 25% Triethanolamine q.s. pH 6.0 10% HCl solution q.s. pH 6.0 Purified water q.s. 100.00

The gel composition was made by combining metronidazole, propylene glycol, niacinamide, beta-cyclodextrin, methylparaben, propylparaben, EDTA disodium, Transcutol® P, and benzyl alcohol in a manufacturing vessel. These components were mixed while heating to 65° C. until a clear solution was obtained. After removal from the heat source, the gelling agent, Carbopol® Ultrez 10 (Lubrizol Corp., Wickliffe, Ohio) was added with continuous mixing until dispersed into the mixture. Mixing was continued until a homogenous gel was formed and the system reached room temperature. The pH was then adjusted to 6.0±0.3 with triethanolamine or HCl solution.

Example 7 Metronidazole 1.25% and 1.5% Gels

Exemplary gel pharmaceutical compositions containing metronidazole at a concentration of 1.25% w/w and 1.5% w/w were produced according to the method of Example 6 by combining the following components, as shown below in Tables 8 and 9, respectively. The solutions were determined to be stable with no evidence of precipitate formation following storage at room temperature for two weeks or at a temperature of 5° C. for one week.

TABLE 8 Components % w/w Metronidazole 1.25 Propylene glycol 5.00 Methylparaben 0.15 Propylparaben 0.05 EDTA sodium 0.05 Ethoxydiglycol 25.00 Benzyl alcohol 1.00 Carbopol ® Ultrez 10 0.50 25% Triethanolamine q.s. pH 6.0 10% HCl solution q.s. pH 6.0 Purified water q.s. 100.00

TABLE 9 Components % w/w Metronidazole 1.50 Propylene glycol 10.00 Methylparaben 0.15 Propylparaben 0.05 EDTA sodium 0.05 Ethoxydiglycol 25.00 Benzyl alcohol 2.00 Carbopol ® Ultrez 10 0.50 25% Triethanolamine q.s. pH 6.0 10% HCl solution q.s. pH 6.0 Purified water q.s. 100.00

Example 8 Metronidazole 1.25% and 1.5% Gels

Exemplary gel pharmaceutical compositions containing metronidazole at a concentration of 1.25% w/w and 1.5% w/w were produced according to the method of Example 6 by combining the following components, as shown below in Tables 10 and 11, respectively. The solutions were determined to be stable with no evidence of precipitate formation following storage at room temperature for two weeks or at a temperature of 5° C. for one week.

TABLE 10 Components % w/w Metronidazole 1.25 PEG 400 5.00 Methylparaben 0.15 Propylparaben 0.05 EDTA sodium 0.05 Ethoxydiglycol 35.00 Ethanol 3.00 Carbopol ® Ultrez 10 0.50 25% Triethanolamine q.s. pH 6.0 10% HCl solution q.s. pH 6.0 Purified water q.s. 100.00

TABLE 11 Components % w/w Metronidazole 1.50 Hexylene glycol 5.00 Methylparaben 0.15 Propylparaben 0.05 EDTA sodium 0.05 Ethoxydiglycol 35.00 Ethanol 3.00 Carbopol ® Ultrez 10 0.50 25% Triethanolamine q.s. pH 6.0 10% HCl solution q.s. pH 6.0 Purified water q.s. 100.00

Various modifications of the above described invention will be evident to those skilled in the art. It is intended that such modifications are included within the scope of the following claims. 

1. An aqueous solution comprising water at a concentration of about 55% w/w or higher, a poorly water-soluble imidazole or triazole chemical compound, and at least two of a polyol, a polyol ether, and a low carbon organic alcohol, wherein the at least two of a polyol, a polyol ether, and a low carbon organic alcohol are present in the aqueous solution in amounts sufficient to provide a synergistic effect on the solubility of the chemical compound in the aqueous solution.
 2. The aqueous solution of claim 1 wherein the imidazole or triazole chemical compound is selected from the group consisting of metronidazole, ketoconazole, itraconazole, miconazole, dimetridazole, secnidazole, omidazole, tinidazole, carnidazole, and panidazole.
 3. The aqueous solution of claim 2 wherein the chemical compound is metronidazole.
 4. The aqueous solution of claim 1 which comprises a polyol and a polyol ether.
 5. The aqueous solution of claim 1 which comprises a polyol and a low carbon organic alcohol.
 6. The aqueous solution of claim 1 which comprises a polyol ether and a low carbon organic alcohol.
 7. The aqueous solution of claim 1 which comprises a polyol, a polyol ether, and a low carbon organic alcohol.
 8. The aqueous solution of claim 7 wherein the polyol is propylene glycol, the polyol ether is diethylene glycol monoethyl ether, and the low carbon organic alcohol is benzyl alcohol.
 9. The aqueous solution of claim 1 wherein the polyol is selected from the group consisting of polyether glycol, propylene glycol, hexylene glycol, and sugar alcohol.
 10. The aqueous solution of claim 9 wherein the polyol is propylene glycol.
 11. The aqueous solution of claim 1 wherein the polyol ether is selected from the group consisting of diethylene glycol monoethyl ether, ethers of pentaerythritol, ethers of alkylene glycol, ethers of a fatty alcohol, and ethers of a sugar.
 12. The aqueous solution of claim 11 wherein the polyol ether is diethylene glycol monoethyl ether.
 13. The aqueous solution of claim 1 wherein the low carbon organic alcohols is selected from the group consisting of ethyl alcohol, propyl alcohol, phenol, and benzyl alcohol.
 14. The aqueous solution of claim 1 wherein the polyol is propylene glycol, the polyol ether is diethylene glycol monoethyl ether, and the low carbon organic alcohol is benzyl alcohol.
 15. The aqueous solution of claim 1 which is stable when stored for a period of two weeks at room temperature or for a period of one week or more at a temperature of 5° C.
 16. A method for making an aqueous solution of a poorly water-soluble imidazole or triazole chemical compound comprising combining in an aqueous fluid the chemical compound and at least two of a polyol, a polyol ether, and a low carbon organic alcohol, wherein the at least two of a polyol, a polyol ether, and a low carbon organic alcohol are combined in the aqueous fluid in amounts sufficient to provide a synergistic effect on the solubility of the chemical compound in the aqueous fluid, and wherein the concentration of water in the aqueous fluid is about 55% w/w or higher.
 17. The method of claim 16 wherein the imidazole or triazole chemical compound is selected from the group consisting of metronidazole, ketoconazole, itraconazole, miconazole, dimetridazole, secnidazole, ornidazole, tinidazole, carnidazole, and panidazole.
 18. The method of claim 17 wherein the chemical compound is metronidazole.
 19. The method of claim 16 wherein a polyol and a polyol ether are combined in the aqueous fluid.
 20. The method of claim 16 wherein a polyol and a low carbon organic alcohol are combined in the aqueous fluid.
 21. The method of claim 16 wherein a polyol ether and a low carbon organic alcohol are combined in the aqueous fluid.
 22. The method of claim 16 wherein a polyol, a polyol ether, and a low carbon organic alcohol are combined in the aqueous fluid.
 23. The method of claim 22 wherein the polyol is propylene glycol, the polyol ether is diethylene glycol monoethyl ether, and the low carbon organic alcohol is benzyl alcohol.
 24. The method of claim 16 wherein the polyol is selected from the group consisting of polyether glycol, propylene glycol, and sugar alcohol.
 25. The method of claim 24 wherein the polyol is propylene glycol.
 26. The method of claim 16 wherein the polyol ether is selected from the group consisting of diethylene glycol monoethyl ether, ethers of pentaerythritol, ethers of alkylene glycol, ethers of a fatty alcohol, and ethers of a sugar.
 27. The method of claim 26 wherein the polyol ether is diethylene glycol monoethyl ether.
 28. The method of claim 16 wherein the low carbon organic alcohol is selected from the group consisting of ethyl alcohol, propyl alcohol, phenol, and benzyl alcohol.
 29. The method of claim 16 wherein the polyol is propylene glycol, the polyol ether is diethylene glycol monoethyl ether, and the low carbon organic alcohol is benzyl alcohol. 